1. Field of the Invention
This invention relates to methods and apparatus for determining formation fluid properties. More particularly, the invention relates to determination of the formation fluid properties using a downhole fluid sampling tool equipped with an NMR module.
2. Background Art
The oil and gas industry has developed various tools capable of determining formation fluid properties. For example, borehole fluid sampling and testing tools such as Schlumberger's Modular Formation Dynamics Testing (MDT™) Tool can provide important information on the type and properties of reservoir fluids in addition to providing measurements of reservoir pressure, permeability, and mobility. These tools may perform measurements of the fluid properties downhole, using sensor modules on board the tools. Alternatively, these tools can withdraw fluid samples from the reservoir that can be collected in bottles and brought to the surface for analysis. The collected samples are routinely sent to fluid properties laboratories for analysis of physical properties that include, among other things, oil viscosity, gas-oil ratio, mass density or API gravity, molecular composition, H2S, asphaltenes, resins, and various other impurity concentrations. However, the laboratory data may not be useful or relevant to the reservoir fluid properties because the samples may have changed properties when brought to surface.
For example, the formation fluid may contain dissolved gas that will separate from liquids when the outside pressure drops. Similarly, the formation fluid may include substances that may precipitate out when the outside temperature drops. In either case, the measured laboratory data may not be relevant to the actual properties of the in situ reservoir fluids. Therefore, it is desirable that formation fluid analysis be performed under downhole conditions.
Several U.S. Patents disclose methods and apparatus for making NMR measurements in a borehole on fluid samples withdrawn from earth formations. For example, U.S. Pat. No. 6,346,813 B1 issued to Kleinberg (the '813 patent) discloses an NMR module on the flowline of the MDT™ tool for determining different fluid properties from magnetic resonance signals. The '813 patent is assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety. U.S. Pat. No. 6,107,796 issued to M. Prammer discloses apparatus and methods for determining the level of contamination in a formation crude oil sample that may be contaminated by oil-based mud filtrate. The method discloses in this patent monitors changes in NMR responses of fluid samples as a function of time while the fluid samples are pumped from the formation into a sampling tool.
Formation fluids often contain several components, each of which likely has a different diffusion property. Therefore, measurement of diffusion coefficients may provide valuable information on the formation fluid properties. Some NMR methods make use of magnetic field gradients to probe the diffusion properties of the formation fluids. For example, U.S. Pat. No. 6,737,864 issued to Prammer et al. discloses an apparatus for making T1 measurements on fluids flowing in the flowline of a fluid sampling tool. This application also discloses a static gradient method for making diffusion measurements on stationary samples. The method of deriving the diffusion data from the NMR measurements assumes a single diffusion constant. However, it is well known that crude oils have a distribution of diffusion coefficients. Thus, it is desirable to have methods that can provide diffusion coefficients of formation fluids without assuming that they have the same diffusion constants.
U.S. Pat. No. 6,111,408 (the '408 patent) issued to Blades et al. discloses methods and apparatus for measuring the relaxation times (T1 and T2) and the diffusion coefficients (D) of fluids in an NMR module of a fluid sampling tool. A method disclosed in this patent uses an electromagnet to generate an oscillating pulse field gradient (PFG) in between refocusing pulses of CPMG pulse sequence. The oscillating PFG is phased locked (synchronized) with the CPMG pulses (see FIG. 5 in the '408 patent). The pulse field gradient de-phases the spins and then is turned off for a period, during which the spins diffuse. Following the diffusion period, the oscillating pulse field gradient is turned on to re-phase the spins followed by a spin-echo. The first spin-echo is then re-focused by a train of radio frequency 180-degree pulses to obtain more spin-echoes. While the phase-locked oscillating PFG pulses are capable of providing diffusion encoding, better methods and apparatus for accomplishing diffusion encoding are desirable.
U.S. Pat. No. 6,346,813 B1 issued to Kleinberg discloses an NMR module for characterizing fluids in a fluid sampling and testing tool, such as the MDT™ tool. This patent discloses methods for relating relaxation times and diffusion coefficients of fluids to viscosity, gas-oil ratio (GOR), and other fluid properties of interest. A related U.S. Pat. No. 5,796,252 issued to Kleinberg et al. (the '252 patent) discloses the use of PFG-CPMG sequence to encode diffusion information. A simple approximation method is then used to obtain a diffusion coefficient from the PFG-CPMG data. The diffusion coefficient is then used to correct for spin echo magnitudes in order to derive more accurate oil volumes in reservoirs. The methods disclosed in the '252 patent also assume a single diffusion coefficient for crude oils.
U.S. Pat. No. 6,462,542 B1 issued to L. Venkataramanan et al. (the '542 patent) discloses “diffusion-editing” pulse sequences. The diffusion information is encoded using a static gradient of the applied magnetic field using a wireline or logging-while-drilling (LWD) NMR logging tool. These pulse sequences are modifications of CPMG sequences. The pulse sequence differs from the CPMG sequence in that one or a few of the early echoes are acquired with a long echo spacing in order to produce diffusion attenuation of the echoes. The remaining echoes are acquired with a short spacing to minimize diffusion effects (D). The '542 patent also discloses an inversion of a physics model using a suite of diffusion-editing pulse sequences that provides 2-dimensional distribution functions of D−T2, T1−T2, and T1/T2−T2.
U.S. Pat. No. 6,570,382 by Hürlimann et al. also discloses “diffusion editing” pulses sequences that may include a pulsed field gradient sequence.
While various NMR apparatus and methods are available for determining formation fluid properties, better methods and apparatus for determining formation fluid properties are still needed.